1. Field of the Invention
The present invention relates to a semiconductor device and a manufacture method thereof, and in particular, to an insulating film provided in a semiconductor device.
2. Description of the Related Art
Silicon nitride films (SiN films) are applied to various parts of a semiconductor device. However, conventional SiN films formed using dichlorosilane (SiH2Cl2:DCS) may create various problems when they are used to manufacture next-generation semiconductor devices.
Possible problems with a next-generation DRAM employing a dual gate will be described by way of example. In a next-generation DRAM, a thick SiN film of about 200 nm thickness is used as a hard mask used to process an electrode. In the case of an SiN film using DCS (DCS-SiN film), the diffusion of boron is enhanced owing to high temperature steps executed after film formation. Consequently, a PMOS device may be degraded. The degradation of the PMOS device attributed to the SiN film can be suppressed to some degree by taking measures to modify integration. However, such measures may degrade the performance of the transistor and are thus difficult to implement. Accordingly, for more essential solutions, SiN films must be developed which do not contribute to degrading PMOS devices.
The degradation of a device associated with a DCS-SiN film can be prevented using an SiN film (TCS-SiN film) used of tetrachlorosilane (SiCl4:TCS). However, the speed at which the TCS-SiN film is formed is low and about one-third of that of the DCS-SiN film. The film formation speed can be increased by changing film formation conditions (film formation temperature, pressure, and others). However, it is actually difficult to increase the film formation speed because of the need to keep the film uniform, prevent film quality from being degraded, suppress dusts, and the like. Consequently, the use of the TCS-SiN film may reduce productivity.
Further, a MONOS type device using a silicon nitride film as a charge storage layer has been proposed as a cell structure of a next-generation flash memory. The MONOS device comprises a semiconductor substrate, a silicon oxide film (a tunnel oxide film or bottom oxide film), a silicon nitride film (a charge storage layer), a silicon oxide film (a top oxide film), and an electrode sequentially stacked together. This is an M—O—N—O—S structure. An electric write of information is carried out by injecting electrons or holes from the semiconductor substrate into the silicon nitride film through the tunnel oxide film.
A problem with the MONOS device is destruction of data resulting from write/erase stress. Further, a problem with NAND type devices is destruction of data resulting from read stress. Non-volatile memories need to retain charges for 10 years after writes/erases have been carried out 100,000 times. At present, however, data are not sufficiently retained.
In the prior art, Jpn. Pat. Appln. KOKOKU Publication No. 2-59632 discloses a structure using two SiN films containing different amounts of hydrogen, as a charge storage layer. In this case, silane and ammonia are used as film formation gases. Specifically, an SiN film having a larger number of Si—H bonds is provided under an SiN film having a smaller number of Si—H bonds to allow data to be more appropriately retained. However, this structure is not always optimum as described later.
Jpn. Pat. Appln. KOKAI Publication No. 9-64205 discloses a structure using an SiN film as a charge storage layer which has a silicon concentration peak near a top surface of the SiN film, while having a nitrogen concentration peak near a bottom surface thereof. For example, DCS and ammonia are used as film formation gases. Specifically, the concentrations of silicon and nitrogen are adjusted by implanting silicon and nitrogen ions into a single layer SiN film. However, the SiN film is a single layer formed using DCS or the like, and this structure is not always optimum.
Jpn. Pat. Appln. KOKOKU Publication No. 5-48631 discloses a structure in which a silicon oxynitride film is formed on a side of a bottom oxide film as a charge storage layer. Such a structure allows data to be more appropriately retained. However, this is not always optimum as described later.
Further, for non-volatile memories such as flash memories, a tunnel insulating film is desired to be thinner in order to accommodate the continuously reduced thickness of devices. If a silicon oxide film or a silicon oxynitride film is used as a tunnel insulating film, a leakage current may be generated by a mechanism known as direct tunneling when a low electric field of at most 5 MV/cm is applied. This hinders data from being appropriately retained.
Thus, to reduce the low-electric-field leakage current, it has been proposed that a silicon nitride film be used as a tunnel insulating film (Non-Volatile Semiconductor Memory Workshop 1998, p. 95–97, and Non-Volatile Semiconductor Memory Workshop 2001, p. 67–69). However, in spite of the excellent initial characteristics of this film, a gradually increasing low-electric-field leakage current called an “SILC (Stress Induced Leakage Current)” may be generated therein as the number of writes/erases increases. Consequently, this film cannot sufficiently retain data.
As described above, the problem with the formation of a silicon nitride film using DCS can be solved using TCS. However, the use of TCS hinders the film formation speed from being increased, thus reducing productivity.
Further, the non-volatile memory device has been proposed which uses a silicon nitride film as a charge storage layer. However, the corresponding conventional structure does not allow data to be sufficiently retained.
Furthermore, it has been proposed that a silicon nitride film be used as a tunnel insulating film of a non-volatile memory device. However, the corresponding conventional structure does not allow data to be sufficiently retained.